This invention relates to high-potential testing of vacuum-type circuit interrupters to determine whether the pressure therein has risen to abnormal levels and, more particularly, relates to this type of testing as applied to a vacuum interrupter that is susceptible to external spark-overs when subjected to high potentials applied between its fully-separated contacts of an effective value equal or greater than a predetermined minimum value sufficient to consistently produce a spark-over between said fully-separated contacts when said interrupter is at atmospheric pressure.
The usual vacuum-type circuit interrupter comprises a highly-evacuated envelope within which there is located a pair of separable contacts. When the contacts are separated during an interrupting operation, an arc is drawn between them; and this arc persists until a natural current zero, at which time it is prevented from reigniting by the high dielectric strength of the vacuum. For the interrupter to function properly, the normal pressure within the envelope must be below a predetermined maximum level.
A conventional way of testing the interrupter to determine whether the pressure therein has risen to an abnormal level is to apply between its fully-separated contacts a high potential which has a sufficient effective value to consistently produce a spark-over between the contacts if the pressure therein has risen to an abnormal value. Such a pressure rise might have resulted from a leak in the envelope of the interrupter. This conventional test method works quite well if the vacuum interrupter has a high voltage rating and thus has sufficient external dielectric strength to prevent an external spark-over when this potential is applied. But if the interrupter has a relatively low voltage rating, and, consequently, considerably less external dielectric strength, application of the above-described high potential may very well produce an external spark-over. This is particularly so if the normal external dielectric strength has been reduced as a result of high humidity or contamination or a combination of these two conditions.
If the person testing the interrupter was not aware that the spark-over was an external one, he might be misled into thinking that the vacuum within the interrupter had been lost, and he might therefore erroneously replace and perhaps even discard the interrupter even though it was perfectly good. Even if he was aware that the spark-over was an external one, he would be frustrated in continuing with the test because of such external spark-overs.
A possible way of alleviating this problem is to use a lower value of high potential for testing the interrupter. This may be satisfactory if the pressure within the interrupter has risen only slightly from its normal range, for example, to the glow level. But if the pressure has risen to atmospheric, or to the neighborhood of atmospheric, this low level of high potential might not be sufficient to produce a spark-over between the contacts due to the higher dielectric strength within the interrupter.